Due to the use of surfactants and other water soluble materials, water-based emulsion polymers (i.e., latices) are known to be water-sensitive. Emulsion polymers typically become hazy under high humidity or wet conditions, and hazy emulsions have been described as "cloudy", "milky" and "white" in the literature. To prevent or reduce haze in emulsion polymers, many approaches have been employed and reported in literature. Such approaches have involved using polymerizable surfactants, surfactant-free emulsion polymerization methods, alternative monomer and initiator selection, pH control of emulsion system, etc.
One use for such emulsion polymers, particularly acrylic-based emulsion pressure sensitive adhesives, is as a bonding or joining layer in laminated or layered constructions. Emulsion adhesive-coated films can be laminated to another surface either under dry conditions, generally referred to as "dry lamination", or under wet conditions using, for example, water. This latter laminating process is generally referred to as "wet lamination." Dry lamination has the advantage of being fast (i.e., takes the least amount of time), but if the acrylic emulsion adhesive is water-sensitive, there can be problems with haze once the finished laminate is exposed to moisture or humidity. Dry laminating methods also are susceptible to entrapment of air bubbles between the adhesive layer and bonded surface. Thus, to obtain optically clear and bubble free lamination in the dry lamination process, high pressure and/or high temperature processing is typically required, such as treating the laminates in an autoclave. Such additional processing often requires specialized and expensive equipment and, when considered for larger articles, is impossible or impractical.
An alternative method for obtaining bubble free lamination is to use a wet lamination process. Such processes are typically used for window glass film laminations. These procedures include spraying water onto window glass surface, then laminating the adhesive coated film onto the wet glass surface. In addition to the fact that wet lamination generally provides bubble free lamination, the film remains repositionable to allow proper placement of the film on the glass surface. Once dry, the adhesive loses its repositionable character and the bond to the glass surface builds to the required strength.
Unfortunately, most acrylic emulsion pressure sensitive adhesives (PSAs) whiten when exposed to water. These emulsion PSAs will regain their original clarity following the wet lamination process. However, the milky white color will return under high temperature and high humidity conditions. This whitening is undesirable in some adhesive applications and is unacceptable in decorative, light management or optical articles the require high clarity under a variety of conditions in the finished product. Thus, an acceptable emulsion PSA useful in the above mentioned applications should possess non-whitening and low haze properties during both the wet lamination process and when exposed to high temperature and high humidity environments.
Solvent-based pressure sensitive adhesives are typically water tolerant, that is, generally water insensitive and can exhibit non-whitening performance under such conditions. Although effective in reducing whitening of adhesives, in many situations and locations the use of solvents is undesirable and/or not permitted. In the adhesives industry, major efforts have been undertaken to remove solvents from formulations due to environmental concerns and regulatory requirements.
Several emulsion acrylic adhesives have been described as water-resistant. However, typically they only exhibit such behavior for only a short period of time. Thus over time, the adhesives become hazy and take on a white or yellowish hue as a result of high heat and humidity conditions. See for examples, U.S. Pat. Nos. 5,420,195, 5,286,843, 4,994,538 and 5,234,736 and PCT Publication No. WO 91/17201.
Thus, what is needed is a water-insensitive, optically clear emulsion acrylic based pressure sensitive adhesive.